Metal oxide pigments are widely used as colorants in the paint, plastics and ceramics industries and they are principally known for their chemical, thermal and weathering stabilities. While the same basic pigment compositions can be used by these industries to color the various medias ranging from complex organic polymers to glass, it is often necessary to adjust the physical properties of each pigment to meet the specifications for its successful commercial application. For example, a ceramic color is composed usually of coarser particles to minimize dissolution in the strong alkaline glazes and thereby maintain color intensity. Conversely, the paint and plastics industries prefer pigments in a relatively finely divided state to permit easy dispersion and optimize properties of gloss, brightness, strength and opacity.
For tinted paints, the necessity of introducing a separate colored pigment can in certain cases introduce weak features as far as their stability to light, heat, solvents and chemicals are concerned resulting in flotation, flocculation, recrystallization, sublimation and chalking. The use of "tinted white" or "titanate pigments" offers a way to overcome the difficulties associated with unstable tinted systems without departing from the whitening, hiding and other valuable pigmentary properties inherent in the "host" element which in this instance is titanium dioxide.
The standard procedure for preparing metal oxide pigments consists of calcining an intimate mixture of oxides or oxide precursors of primarily transition metals. Color properties develop from the formation of solid solutions containing transition metals having colored ions. Colors produced by calcining essentially white pigments with small additions of foreign elements to produce stable tinted pigments have been commercially developed over the last 30 years. For example, titanium dioxide becomes colored when calcined with minor amounts of transition metals. For tinted paints, the necessity of introducing a separate colored pigment can in certain cases introduce weak feature as far as their stability to light, heat, solvents and chemicals are concerned leading to flotation, flocculation, recrystallization, sublimation and chalking. The use of "tinted white" with unstable tinted systems without departing from the whitening, hiding and other valuable pigmentary properties inherent in the "host" element which in this case is titanium dioxide.
A large number of pigments in which the major component is a white pigment such as titanium dioxide which accepts minor additions of intensely colored metal ions to produce pastel colorants are described in U.S. Pat. No. 3,022,186. This patent describes solid solutions resulting from a large number of combinations of metal oxides or fluorides as guest components in a number of host lattices, but principally rutile titanium dioxide. The pigments prepared or disclosed in U.S. Pat. No. 3,022,186, while possessing the desired color values, are generally deficient in some physical characteristics such as texture, i.e., ease of dispersibility, gloss or opacity necessary for commercial use in the paint and plastics industries. Among the various of metals described in the '186 patent which may be added to the rutile lattice forming solid solutions are chromium, antimony, strontium, cobalt, tungsten, lithium and cerium.
Example 7 describes a reddish light brown pigment having rutile structure which is prepared by calcining a mixture of anatase, CoCO.sub.3 and H.sub.2 WO.sub.4. An orange pigment is prepared in Example 20 from anatase, CoCO.sub.3, NiO and H.sub.2 WO.sub.4. Examples 129 and 130 describe the preparation of a yellowish-whitish grey pigment from anatase or rutile, H.sub.2 WO.sub.4 and LiF.
Reissue U.S. Pat. No. Re. 21,427 describes the process for forming titanium dioxide pigments by combining the titanium dioxide with a compound of a heavy metal. Examples of heavy metals listed in Col. 1, lines 21-25 include vanadium, chromium, iron, cobalt, nickel, manganese and copper.
U.S. Pat. No. 3,956,007 describes antimony-free yellow pigments prepared by calcining anatase with a specified amount of nickel, tungsten, zinc, lithium, cerium and magnesium compounds.
U.S. Pat. No. 5,006,175 describes high infra red reflectory brown rutile pigments comprising a solid solution of titanium dioxide containing within the rutile lattice, manganese, tungsten, lithium and cerium.
In recent years there has been increasing emphasis on developing pigments which do not contain certain metals such as antimony, chromium, arsenic, bismuth, cadmium, selenium, mercury, soluble barium, etc. Thus it is desirable to produce buff pigments which are free of such metals and which have the same or better qualities than the previously known pigments.